m 


Notes  on  Rigging 

—  FOR-  ^^== 

Air  Mech  anics 


WASHINGTON 
Press  of  Gibson  Bros.,  Inc. 


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NOTES  ON  RIGGING  FOR  AIRPLANE 
MECHANICS. 


IMPORTANCE  OF  GOOD  RIGGING. 

It  is  impossible  to  exaggerate  the  importance  of 
CARE  and  ACCURACY  in  rigging.  The  pilot's  life, 
the  speed  and  climb  of  the  airplane,  its  control  and 
general  efficiency  in  fhght,  and  its  duration  as  a  useful 
machine  all  depend  upon  the  rigger.  Consider  that 
while  the  engine  may  fail  the  pilot  may  still  glide 
safely  to  earth  BUT,  if  the  airplane  fails,  then  all  is 
lost.  The  responsibility  of  the  rigger  is,  therefore, 
very  great,  and  he  should  strive  to  become  a  sound  and 
reliable  expert  on  all  matters  relatmg  to  his  art— for  an 
art  it  is,  and  one  bound  to  becOm^ncjpa^ingly  impor- 
tant as  time  passes. 

FLIGHT. 

First  of  all  he  must  have  a  sound  idea  of  flight  and 
stability.  Flight  is  secured  by  driving  through  the 
air  a  surface  or  surfaces  inclined  to  the  direction  of 
motion.  Such  inclination  is  called  the  ANGLE  OF 
INCIDENCE.    (See  Fig.  1.) 


Side  of  <5t/r-foce 
/Vt P/r-ec^tor?  of  Moi-ior? 
A- /in^lt  of  IncK^ertce 

r-iG.  1  ■  - 


Lift. 

In  this  way  the  surfaces,  that  is,  the  lifting  planes, 
secure  a  lift  from  the  air,  and,  when  the  speed  through 
the  air  is  sufficient,  the  lift  will  become  greater  than 

1 


2 

the  weight  of  the  airplane,  which  must  then  rise.  Bear 
in  mind  that  the  hft  is  ahvaj^s  trying  to  collapse  the 
planes  upwards. 

Drift. 

The  resistance  of  the  air  to  the  passage  of  the  air- 
plane is  known  as  drift,  and  this  is  overcome  by  the 
propeller  thrust,  which  thrusts  the  airplane  through 
the  air  and  so  overcomes  the  drift.  Bear  in  mind  that 
the  drift  is  always  trying  to  collapse  the  plane  back- 
wards.   (See  Fig.  2.)' 


Thus  you  will  see  by  this  diagram  that  there  are  f  • 
forces  to  consider.  The  LIFT  which  is  opposed  to  the 
WEIGHT,  and  the  THRUST  which  is  opposed  to  the 
DRIFT.  The  lift  is  useful — the  drift  is  the  reverse  of 
useful.  The  proportion  of  lift  to  drift  is  known  as  the 
LIFT-DRIFT  RATIO.  This  is  of  paramount  impor- 
tance for  upon  it  depends  the  efficiency  of  the  airplane. 
In  rigging  an  airplane  the  greatest  care  must  be  taken 
to  PRESERVE  THE  LIFT-DRIFT  RATIO.  Always 
keep  that  in  mind. 


3 


Angle  of  Incidence. 

The  angle  of  incidence  is  the  incHnation  of  the  hfting 
surfaces.  If  the  angle  of  incidence  is  increased  over  the 
angle  specified  in  your  rigging  instructions  then  both 
the  lift  and  the  drift  are  increased  also — and  the  drift 
is  increased  in  greater  proportion  than  the  lift.  If, 
however,  the  angle  of  incidence  is  decreased,  then  the 
lift  and  the  drift  are  decreased  and  the  lift  decreases  in 
greater  proportion  than  does  the  drift.  You  see  then 
that  in  each  case  the  efficiency  is  lessened,  because  the 
proportion  of  lift  to  drift  is  not  so  good  as  would  other- 
wise be  the  case. 

Balance. 

The  whole  weight  of  the  airplane  is  balanced  upon, 
or  slightly  forward  of,  the  center  of  the  lift.  If  the 
weight  is  too  far  forward  then  the  machine  is  nose 
heavy.  If  the  weight  is  too  far  behind  the  centre  of 
the  lift  then  the  airplane  is  tail  heavy. 

GET  A  GOOD  UNDERSTANDING  OF  THE 
ABOVE  BEFORE  GOING  ANY  TURTHER. 

Stability. 

By  the  stability  of  the  airplane  is  meant  the  tendency 
of  the  airplane  to  remain  upon  an  even  keel,  and  to 
keep  its  course;  that  is  to  say,  not  to  fly  one  wing  down, 
tail  down,  or  nose  down,  or  to  try  and  turn  off  its  course. 

Directional  Stability. 

By  directional  stability  is  meant  the  natural  ten- 
dency of  the  airplane  to  remain  upon  its  course.  If 
this  did  not  exist  it  would  be  continually  trying  to  turn 
to  the  right  or  to  the  left,  and  the  pilot  would  not  be 
able  to  control  it.  For  the  airplane  to  have  directional 
stability  it  is  necessary  for  it  to  have,  in  effect,  more 
keel  surface  behind  its  turning  axis  than  there  is  in 
front  of  it. 

By  keel  surface  is  meant  everything  you  can  see 
when  you  look  at  the  airplane  from  the  side  of  it — the 
sides  of  the  body,  landing  gear,  wires,  struts,  etc. 
Directional  stability  is  sometimes  known  as  "weather- 


4 

cock"  stabilit3^    You  know  what  would  happen  if,  in 
the  case  of  the  weathercock,  there  was  too  much  keel 
surface  in  front  of  its  turning  axis,  which  is  the  point  ^ 
upon  which  it  is  pivoted.    It  would  turn  the  wrong 
way.    That  is  just  how  it  is  with  an  airplane. 

Directional  stabiUty  will  be  badly  affected  if  there  is 
more  drift,  i.  e.,  resistance,  on  one  side  of  the  airplane 
than  there  is  on  the  other  side.  This  may  be  caused 
as  follows.: 

1.  The  angle  of  incidence  of  the  mam  planes  or  the 
tail  plane  may  be  wrong.  If  the  angle  of  incidence  on 
one  side  of  the  machine  is  not  what  it;  should  be,  there 
will  be  a  difference  in  the  drift  betw  een  the  two  sides 
of  the  airplane,  with  the  result  that  it  will  turn  off  its 
course. 

2.  If  the  ahgnment  of  the  fuselage  fin  in  front  of  the 
rudder,  is  not  absolutely  correct,  that  is  to  say,  if  it 
is  turned  a  little  to  the  left  or  to  the  right,  instead  of 
being  in  line  with  the  center  of  the  machine  and  dead 
on  in  the  direction  of  flight,  it  will  act  as  an  enormous 
rudder  and  cause  the  machine  to  turn  off  its  course. 

3.  If  the  dihedral  angle  is  wrong,  that  may  have  a 
bad  effect.  It  may  result  in  the  propeller  not  thrusting 
from  the  center  of  the  drift,  in  which  case  it  will  pull 
the  macliine  a  little  sidew^ays,  and  out  of  its  course. 

4.  If  the  struts  and  stream  line  wires  are  not  adjusted 
to  be  dead  on  in  the  hne  of  flight,  then  they  will  pro- 
duce additional  drift  on  then  side  of  the  airplane,  with 
the  result  that  it  will  turn  off  its  course. 

5.  There  is  still  one  other  reason  why  the  airplane 
may  be  directionally  bad,  and  that  is  DISTORTED 
SURFACES.  You  must  understand  that  the  planes 
are  "cambered,"  that  is,  curved  to  go  through  the  air 
with  the  least  possible  drift.  If,  perhaps,  owing  to  the 
leading  edge,  spars  or  trailing  edge  getting  bent,  the 
curvature  is  changed,  it  will  result  in  changing  the 
amount  of  drift  on  one  side  of  the  airplane,  which  will 
then  have  a  tendency  to  turn  off  its  course. 

Lateral  StabOity. 

By  lateral  stability  is  meant  the  sideways  balance  of 
the  machine.    The  only  possible  thing  that  can  make 


5 

the  machine  fly  one  wing  down  is  that  there  is  more  lift 
on  one  side  than  on  the  other.  That  may  be  due  to 
the  following  reasons: 

1.  The  angle  of  incidence  may  be  wrong.  If  the 
angle  of  incidence  is  too  great,  then  it  will  produce  more 
lift  than  on  the  other  side  of  the  machine,  and  if  the 
angle  of  incidence  is  too  small,  then  it  will  produce  less 
lift  than  on  the  other  side,  the  result  being  that  in 
either  case  the  machine  will  try  to  fly  one  wing  down. 


6 

2,  Distorted  surfaces. — If  the  planes  are  distorted, 
then  their  camber  or  curvature  is  changed  and  the  lift 
will  not  be  the  same  on  both  sides  of  the  airplane,  and 
that,  of  course,  will  cause  it  to  fly  one  wing  down. 
(See  Fig.  3.) 

Longitudinal  Stability. 

By  longitudinal  stabiUty  is  meant  the  fore  and  aft 
balance.  If  that  is  not  perfectly  right  then  the  machine 
will  try  to  fly  nose  down  or  tail  down.  This  may  be 
due  to  the  following  reasons: 

1.  The  stagger  may  be  wrong.  The  top  plane  may 
have  drifted  back  a  little  and  this  will  probably  be 
due  to  some  of  the  wires  having  elongated  their  loops 
or  having  pulled  the  fittings  into  the  wood.  If  the 
top  plane  is  not  staggered  forward  to  the  correct  degree, , 
then  that  means  that  the  whole  of  its  lift  is  moved 
backwards  and  it  will  then  have  a  tendency  to  lift  up 
the  tail  of  the  machine  too  much.  In  such  a  case  the 
machine  would  be  said  to  be  "nose  heavy."  A  Hnch 
error  in  the  stagger  will  make  a  very  considerable 
difference  to  the  longitudinal  stabiUty. 

2.  If  the  angle  of  incidence  of  the  main  planes  is  not 
right,  that  will  have  a  bad  effect.  If  the  angle  is  too 
great  it  will  produce  an  excess  of  lift,  and  that  will  lift 
up  the  nose  of  the  machine  and  result  in  its  trying  to 
fly  tail  down.  If  the  angle  is  too  small,  it  will  produce 
a  decreased  Uft  and  the  machine  will  try  to  fly  nose 
down. 

3.  When  the  machine  is  longitudinally  out  of  balance 
the  usual  thing  for  the  rigger  is  to  rush  to  the  tail  plane, 
thinking  that  its  adjustment  relative  to  the  fuselage 
must  be  wrong.  This  is,  indeed,  sometimes  the  case, 
but  it  is  the  least  hkely  reason.  It  is  much  more  likely 
to  be  one  of  the  first  two  reasons  given  or  the  one  given 
below : 

The  fuselage  may  have  been  ivarped  upwards  or  down- 
wards, thus  giving  the  tail  plane  an  incorrect  angle  of 
incidence.  If  the  tail  plane  has  too  much  angle  of 
incidence  it  will  make  it  lift  too  much  and  the  machine 
will  be  ''nose  heavy."  If  the  tail  plane  has  too  little 
angle  of  incidence  then  it  will  not  lift  enough,  and  the 
machine  will  be  "tail  heavy." 


7 

4.  If  the  above  three  points  are  all  correct,  then  there 
is  a  possibility  of  the  tail  plane  itself  having  assumed  a 
wrong  angle  of  incidence,  in  which  case  it  must  be  cor- 
rected. In  such  event,  if  the  machine  is  nose  heavy, 
the  tail  plane  should  be  given  a  small  angle  of  incidence. 
If  the  machine  is  tail  heavy  then  the  tail  plane  must  be 
given  a  larger  angle  of  incidence,  BUT  be  careful  not 
to  give  the  tail  plane  too  great  an  angle  of  incidence, 
because  the  longitudinal  stabiUty  of  the  airplane 
entirely  depends  on  the  tail  plane  being  set  at  a  much 
smaller  angle  of  incidence  than  the  main  plane,  and  if 
you  cut  the  difference  down  too  much  the  machine 
will  become  uncontrollable  longitudinally.  Some- 
times the  tail  plane  is  set  on  the  machine  at  the  same 
angle  of  incidence  as  the  main  plane,  but  it  actually 
engages  the  air  at  a  lesser  angle  owing  to  the  air  being 
deflected  downwards  by  the  main  planes,  thus  (see 
Fig.  4): 


Maiff  Planes  fl^ackir?^  flir 


Fig.  4- 


Propeller  Torque. 

Owing  to  propeller  torque  the  airplane  has  a  ten- 
dency to  turn  over  sideways  in  the  opposite  direction 


8 

to  which  the  propeller  revolves.  In  some  machines 
this  tendency  is  rather  marked,  and  it  is  offset  by 
increasing  the  angle  of  incidence  on  the  side  tending  to 
fall^  and  in  some  cases  by  also  decreasing  the  angle  of 
incidence  of  the  side  tending  to  rise.  In  this  way  more 
lift  is  secured  on  one  side  of  the  machine  than  on  the 
other,  and  so  the  tendency  to  overtm-n  is  corrected. 
It  is  common  practice  to  offset  propeller  torque  by 
merelv  increasing  the  angle  of  incidence  on  one  side. 
It  is  far  better,  however,  to  give  half  such  increase  on 
one  side,  at  the  same  time  making  a  similar  decrease 
on  the  other  side.  In  this  way  the  angle  is  nearer  the 
normal  angle  and  the  efficiency  of  the  airplane  is  not  so 
much  disturbed. 

Wash  In. 

Wash-In. — When  the  angle  of  incidence  is  increased, 
that  is  called  "Wash-IN." 

Wash-Out. — When  the  angle  of  incidence  is  decreased, 
then  that  is  called  "Wash-OUT." 

Sometimes  a  wash-out  is  given  to  both  sides  of  the 
main  plane.  This  decreases  the  di-ift  towards  the  wing 
tips,  and  consequently  decreases  the  effect  of  gusts 
upon  them.  It  also  renders  the  ailerons  more  effective 
(see  Figs.  5  and  6). 


^/////////////////////7m 


Stresses  and  Strains. 

In  order  to  rig  a  machine  intelligently  it  is  necessary 
to  have  a  correct  idea  of  the  work  every  wire  and  every 
part  of  the  airplane  is  doing. 


9 

The  work  the  part  is  doing  is  known  as  STRESS. 
If,  owing  to  undue  stress,  the  material  becomes 
distorted,  then  such  distortion  is  known  as  STRAIN. 


Fi6.  6. 


Compression. 

The  simple  stress  of  compression  produces  a  crushing 
strain.  As  an  example,  the  interplane  and  fuselage 
struts. 

Tension. 

The  simple  stress  of  tension  results  in  the  strain  of 
elongation.    As  an  example,  all  the  wires. 

Bending. 

The  compound  stress  of  bending  is  composed  of  both 
tension  and  compression.  Now  we  will  suppose  we  are 
going  to  bend  a  piece  of  wood.  Before  being  bent  it 
will  have  the  following  appearance  (see  Fig.  7) : 


You  see  that  the  top  line,  the  bottom  line,  and  the 
center  line  are  all  of  the  same  length. 

Now  we  will  bend  it  right  round  into  a  circle,  thus 
See  Fig.  8).  . 


10 

The  center  line  is  still  the  same  length  as  it  was 
before  being  bent,  but  you  will  note  that  the  top  line, 
being  on  the  outside  of  the  circle,  must  now  be  longer 
than  the  center  line.  That  can  only  be  due  to  the 
strain  of  elongation.  That  is  produced  by  the  stress  of 
tension,  so  you  see  that  the  wood  between  the  center 
Une  and  the  line  on  the  outside  of  the  circle  is  in 
tension,  and  the  greatest  tension  is  on  the  outside  of  the 
circle,  because  there  the  elongation  is  greatest. 


You  will  notice  that  the  line  on  the  inside  of  the  circle, 
which  before  being  bent  was  the  same  length  as  the 
center  line,  must  now  be  shorter  because  it  is  nearest 
to  the  center  of  the  circle.  That  can  only  be  due  to  the 
strain  of  crushing.  That  can  only  be  produced  by  a 
state  of  compression,  so  you  see  that  the  wood  between 
the  center  line  and  the  inside  line  is  in  compression, 
and  the  greatest  compression  is  nearest  to  the  inside  of 
the  circle,  because  there  the  crushing  effect,  i.  e.,  the 
strain,  is  greatest. 


11 

By  this  you  will  see  that  the  wood  near  the  center 
line  is  doing  the  least  work,  and  that  is  why  it  is  possible 
to  hollow  out  the  center  of  spars  and  struts  ^without 
unduly  weakening  them.  In  this  way  25  to  33  per 
cent  of  the  weight  of  wood  in  an  airplane  is  saved. 

Shear. 

Shear  stress  is  such  that  when  the  material  breaks 
under  it,  one  part  slides  over  the  other.  As  an  example, 
the  locking  pins.  Some  of  the  bolts  are  in  a  state  of 
shear  stress  also  because,  in  some  cases,  there  are  lugs 
underneath  the  bolt  heads  from  which  wires  are  taken. 
Owing  to  the  tendency  of  the  wire  the  lug  is  exerting  a 
sideways  pull  on  the  bolt  and  trying  to  break  it  in  such 
a  way  as  to  make  one  part  of  it  slide  over  the  other. 

Torsion. 

Stress  of  torsion. — This  is  a  twisting  stress  composed 
of  compression,  tension,  and  shear  stress.  As  an 
example,  the  propeller  shaft  and  crank  shaft  of  the 
engine. 

NATURE  OF  WOOD  UNDER  STRESS. 

Wood,  for  its  weight,  takes  the  stress  of  compression 
the  best  of  all.  For  instance,  a  walking  stick  of  about 
half  pound  in  weight,  will,  if  kept  perfectly  straight, 
probably  stand  up  to  a  compression  stress  of  a  ton  or 
more  before  crushing,  whereas  if  the  sam^e  stick  is  put 
under  a  bending  load  it  will  probably  collapse  to  a 
stress  of  not  more  than  about  50  pounds.  That  is  a 
very  great  difference  and  since  weight  is  of  greatest 
importance  in  an  airplane,  the  wood  must,  as  far  as 
possible,  be  kept  in  a  state  of  direct  compression.  This 
it  will  do  safely  as  long  as  the  following  conditions  are 
carefully  observed : 

Conditions  to  be  Observed. 

1.  All  the  spars  and  struts  must  be  perfectly  straight 
(see  Fig.  9). 

This  diagram  shows  a  section  through  an  interplane 
strut.    If  it  is  to  be  prevented  from  bending,  then  the 


12 

stress  of  compression  must  be  equally  disposed  around 
the  center  of  strength.  If  it  is  not  straight,  then  there 
will  be  more  compression  on  one  side  of  the  center  of 
strength  than  on  the  other  side.  That  is  a  step  towards 
getting  compression  on  one  side  and  tension  on  the 
other  side,  in  which  case  it  will  be  forced  to  take  a 
bending  stress  for  which  it  is  not  designed. 

Even  if  it  does  not  break  it  will,  in  effect,  become 
shorter,  and  thus  throw  out  of  adjustment  all  the  wires 
attached  to  the  top  and  bottom  of  it,  with  the  result 
that  the  flight  efficiency  of  the  airplane  will  be  lessened, 


 I 


besides  an][undue  and  dangerous  jstress^j^being^Ithrown 
upon  other  wires  (see  Fig.  10). 

2.  Struts  and  sparsTmust  be  symmetrical.  That  is 
meant  that  the  cross  sectional  dimensions  must  be 
correct,  as  otherwise  there  will  be  bulging  places  on 
the  outside,  with  the  result  that  the  stress  will  not  be 
evenly  disposed  around  the  center  of  strength,  and  a 
bending  stress  will  be  produced. 

3.  Struts,  spars,  etc.,  must  he-undamaged.  Remem- 
ber that,  from  what  has  been  said  about  bending  stresses, 
the  outside  fibers  of  the  wood  are  doing  by  far  the  most 
work.  If  these  get  bruised  or  scored,  then  the  strut 
or  spar  suffers  in  strength  much  more  than  one  might 
think  at  first  sight,  and  if  it  ever  gets  a  tendency  to 
bend  it  is  likely  to  go  at  that  point. 


13 

4.  The  wood  must  have  a  good  clear  grain  with  no 
cross-grain,  knots  or  shakes.  Such  blemishes  mean  that 
the  wood  is  in  some  places  weaker  than  in  other  places, 
and  if  it  has  a  tendency  to  bend,  then  it  will  go  at  those 
weak  points. 

5.  The  struts,  spars,  etc.,  must  be  properly  bedded 
into  their  sockets  or  fittings.  To  begin  with,  they  must 
be  a  good  ijushing  or  gentle  tapping  fit.  They  must 
never  be  driven  with  a  heavy  hammer.  Then,  again, 
they  must  bed  well  down,  all  over  their  cross-sectional 
area;  otherwise  the  stress  of  compression  will  be  taken 
on  one  part  of  the  cross-sectional  area  with  the  result 
that  it  will  not  be  evenly  disposed  around  the  center 
of  strength,  and  that  will  produce  a  bending  stress. 


Fic  10. 


The  bottom  of  the  strut  or  spar  should  be  covered  with 
some  sort  of  paint,  bedded  into  the  socket  or  fitting, 
and  then  withdrawn  to  see  if  the  paint  has  stuck  all 
over  the  bottom  of  the  fitting. 

6.  The  atmosphere  is  sometimes  much  damper  than 
at  other  times,  and  this  causes  the  wood  to  expand  and 
contract  appreciably.  This  would  not  matter  but  for 
the  fact  that  it  does  not  expand  and  contract  uniformly, 
but  becomes  unsymmetrical,  i.  e.,  distorted.  This 
should  be  minimised  by  well  varnishing  the  wood  to 
keep  the  moisture  out  of  it. 


14 

Function  of  Interplane  Struts. — These  struts  have  to 
keep  the  planes  apart,  but  this  is  only  part  of  their 
work.  They  must  keep  the  planes  apart  so  that  the 
latter  are  in  their  correct  attitude.  That  is  only  so 
when  the  spars  of  the  bottom  plane  are  parallel  with 
those  of  the  top  plane.  The  chord  of  the  top  plane 
must  also  be'parallel  to  the  chord  of  the  bottom  plane. 
If  that  is  not  so  then  one  plane  will  not  have  the  same 
angle  of  incidence  as  the  other  one.  You  may  think 
that  all  you  have  to  do  is  to  cut  all  your  struts  the  same 
length,  but  that  is  not  the  case. 

Sometimes,  as  illustrated  in  Fig.  11,  the  rear  spar 
is  not  so  thick  as  the  main  spar,  and  it  is  then  necessary 
to  make  up  for  that  lack  of  thickness  by  making  the 
rear  struts  correspondingly  longer.  If  that  is  not  done, 
then  the  top  and  bottom  chords  will  not  be  parallel, 
and  the  top  and  bottom  planes  will  have  different  angles 
of  incidence.  Also  the  sockets  or  fittings  or  even  the 
spars  upon  which  they  are  placed  sometimes  vary  in 
thickness,  and  this  must  be  offset  by  altering  the  length 
of  the  struts.  The  proper  way  to  proceed  in  order  to 
make  sure  that  everything  is  right  is  to  measure  the 
distance  between  the  top  and  bottom  spars  by  the  side 
of  each  strut  end;  if  that  distance,  or  "gap"  as  it  is 
called,  is  not  as  specified  in  your  rigging  diagram, 
make  it  correct  by  changing  the  length  of  the  strut. 
When  measuring  the  gap  between  the  top  and  bottom 
spars  always  he  careful  to  measure  from  the  center  of 
the  spar,  as  it  may  be  set  at  an  angle,  and  the  rear  of 
the  spar  may  be  considerably  lower  than  its  front  (see 
Fig.  11). 

Boring  Holes  in  Wood. — It  is  a  strict  rule  tha-t  no 
spar  may  be  used  which  has  an  unnecessary  hole  in  it. 
Before  boring  a  hole  its  position  must  be  passed  on 
by  whoever  is  in  charge  of  the  shop.  The  hole  should 
be  of  a  size  that  the  bolt  can  be  pushed  in,  or,  at  any 
rate,  not  more  than  gently  tapped.  Bolts  must  not  be 
hammered  in,  as  it  may  split  the  spar.  On  the  other 
hand,  a  bolt  must  not  be  slack  in  the  hole,  as  in  such  a 
case  it  may  work  sideways  and  spht  the  spar,  not  to 
speak  of  throwing  out  of  adjustment  the  wires  leading 
from  the  lug  or  socket  under  the  bolt  head. 


15 

Washers. — Under  the  bolt  head,  and  also  under  the 
nut,  a  washer  must  be  placed;  a  very  large  washer 
compared  with  any  other  form  of  engineering.  This 
is  to  disperse  the  stress  over  a  large  area  of  the  wood, 
otherwise  the  washer  may  be  pulled  into  the  wood  and 
weaken  it,  besides  possibly  throwing  out  of  adjustment 
the  wires  attached  to  the  bolt  or  fitting. 

Locking. — Now  as  regards  locking  the  bolts.  If 
spht  pins  are  used,  be  sure  to  see  that  they  are  used  in 
such  a  way  that  the  nut  cannot  possibly  unscrew.  If 
it  is  locked  by  burring  over  the  bolt,  do  not  use  a  heavy 


Fig.  11 

hammer  and  try  to  spread  the  whole  head  of  the  bolt. 
That  might  damage  the  woodwork  inside  the  plane. 
Use  a  small,  hght  hammer,  and  gently  tap  around  the 
edge  of  the  bolt  until  it  is  burred  over. 

Turnhuckles. — A  turnbuckle  is  composed  of  a  cen- 
tral barrel  into  each  end  of  which  is  screwed  an  eye-bolt. 
Wires  are  taken  from  the  ends  of  the  eyebolts,  and  so, 
by  turning  the  barrel,  the  wires  can  be  adjusted  to  their 
proper  tension.  Eyebolts  must  be  a  good  fit  in  the 
barrel;  that  is  to  say:  not  too  slack  and  not  very  tight. 

There  is  a  rule  that  they  must  be  screwed  into  the 
barrel  for  a  distance  equal  to  not  less  than^twice  their 


16 


diameter,  but  it  is  better  to  screw  them  in  a  good  deal 
more  than  that.  Now  about  turning  the  barrel  to  get 
the  right  adjustment.  The  barrel  looks  solid,  but 
really  it  is  hollowed  out  and  is  much  more  frail  than  it 
appears.  For  that  reason  it  must  NOT  be  turned  by 
seizing  it  with  pliers,  as  that  may  distort  it  and  spoil 
the  bore.  The  proper  method  is  to  pass  a  piece  of  wire 
through  the  hole  in  the  center  and  use  that  as  a  lever. 
When  you  have  got  the  correct  adjustment,  you  niust 
lock  the  turnbuckle  to  prevent  it  from  unscrewing. 
It  is  quite  possible  to  lock  it  in  such  a  way  as  to  allow 
it  to  unscrew  a  quarter  or  half  a  turn,  and  that  will 
throw  the  wires  out  of  the  very  fine  adjustment  neces- 
sary. The  proper  way  is  to  use  the  locking  wire  in 
such  a  way  as  to  oppose  the  tendency  of  the  turn- 
buckle  to  unscrew,  thus  (see  Fig.  12) : 

Ti;gpiBiycK/.t- 


Internal  Turnhuckles. — Turnbuckles  on  the  internal 
wires  inside  the  planes  must  be  well  greased  and  served 
round  with  adhesive  tape. 


WIRES. 

The  following  points  must  be  carefully  observed 
where  wire  is  concerned: 

1.  Quality. — It  must  not  be  too  hard  or  too  soft. 
An  easy  practical  way  of  learning  to  know  the  quality 
of  wire  is  as  follows: 

Take  three  pieces  of  wire  all  of  the  same  gauge  and 
each  about  a  foot  in  length.  One  piece  should  be  too 
soft,  another  piece  should  be  too  hard,  and  the  third 
piece  of  the  right  quality.  Fix  them  in  a  vise  about 
an  inch  apart  and  in  a  vertical  position,  and  with  the 
light  from  a  window^  shining  upon  them.  Burnish 
them  if  necessary,  and  you  will  see  a  bar  of  light 
reflected  from  each  wire. 


17 


Now  bend  the  wires  over  as  far  as  possible.  Where 
the  soft  wire  is  concerned  it  will  squash  out  at  the  bend, 
and  you  will  see  this  because  the  band  of  light  will  have 
broadened  out  there.  In  the  case  of  the  wire  which 
was  too  hard  the  band  of  light  will  broaden  out  very 
little  at  the  turn,  but  if  you  look  carefully  you  will  see 
some  little  cracks  or  roughness  on  the  surface.  In  the 
case  of  the  wire  of  the  right  quality,  the  band  of  light 
may  have  broadened  out  a  very  little  at  the  turn,  but 
there  will  be  NO  cracks  or  roughness  on  it  at  all. 

By  making  this  experiment  two  or  three  times  you 
will  soon  learn  to  know  good  wire  from  bad,  and  also 
learn  to  know  the  strength  of  hand  necessary  to  bend 
the  right  quality. 

2.  It  Must  Not  he  Damaged. — That  is  to  say,  it  must 
be  unkinked,  rustless  and  unscored. 

3.  As  Regards  Keeping  Wire  in  Good  Condition. — 
Where  the  outside  wires  are  concerned  they  should  be 
kept  well  greased  or  oiled,  especially  where  bent  over 
at  the  ends.  In  the  case  of  internal  bracing  wires 
which  cannot  be  reached  for  the  purpose  of  re-greasing 
them,  you  will  prevent  them  from  rusting  by  painting 
them  with  lacquer.  You  must  be  very  careful  to  see 
that  the  wire  is  perfectly  clean  and  dry  before  painting 
with  lacquer.  A  greasy  finger  mark  is  sufficient  to 
keep  the  lacquer  from  sticking  to  the  wire.  In  such  a 
case  there  will  be  a  little  space  between  the  lacquer  and 
the  wire.  Air  can  enter  there  and  cause  the  wire  to 
rust  under  the  lacquer. 

Tension  of  Wires. 

The  tension  to  which  you  adjust  the  wires  is  of  the 
greatest  importance.  All  the  wires  on  an  airplane 
should  be  of  the  SAME  TENSION,  otherwise  the  air- 
plane will  quickly  become  distorted  and  fly  badly.  As 
a  rule  the  wires  are  tensioned  too  much.  The  tension 
should  be  SUFFICIENT  TO  KEEP  THE  FRAME- 
WORK RIGID.  Anything  more  than  that  changes  the 
factor  of  safety,  throws  various  parts  of  the  framework 
into  undue  compression,  pulls  the  fittings  into  the  wood, 
and  will,  in  the  end,  distort  the  whole  framework  of  the 
airplane. 


18 

Only  experience  will  tell  you  what  tension  to  employ 
and  assist  you  in  making  all  the  wires  of  the  same  ten- 
sion. Learn  the  construction  of  the  various  types  of 
airplanes,  the  work  the  various  parts  do,  and  cultivate 
a  touch  for  tensioning  the  wires  by  constantly  handling 
them. 

Wires  with  No  Opposition  Wires. 

In  some  cases  you  will  find  wires  having  no  opposition 
wires  as  the  overhang  in  the  Curtiss  machines.  In  such 
cases  be  extremely  careful  not  to  tighten  such  wires 
beyond  taking  up  the  slack.  They  must  be  a  little 
slack,  as  otherwise  they  wiU  distort  the  top  spar  down- 
ward. That  will  change  the  camber  (curvature)  of  the 
plane  and  result  in  changing  both  the  lift  and  drift  at 
that  part  of  the  plane.  Such  a  condition  will  cause  the 
machine  to  lose  its  directional  stability  and  also  to  fly 
one  wing  down. 

Wire  Loops. 

,  Wires  often  bent  over  at  the  end  in  the  form  of  a  loop. 
These  loops,  even  when  made  perfectly,  have  a  ten- 
dency to  elongate,  thus  spoiling  the  adjustment  of  the 
wires.  Great  care  should  be  taken  to  minimise  this  as 
much  as  possible.  The  rules  to  be  observed  are  as 
follows: 

1.  The  size  of  the  loop  should  be  as  small  as  possible 
within  reason.  By  that  is  meant  that  it  should  not  be 
so  small  as  to  create  the  possibility  of  the  wire  breaking. 

2.  The  shape  of  the  loop  must  be  symmetrical. 

3.  The  loop  should  have  good  shoulders  in  order  to 
prevent  the  ferrule  from  slipping  up.  At  the  same  time 
the  shoulders  should  have  no  angular  points. 

4.  When  the  loop  is  finished  it  should  be  undamaged, 
and  it  should  not  be,  as  is  often  the  case,  badly  scored 
(see  Fig.  13). 

Strained  Wire  Cables. 

No  splice  should  be  served  with  twine  until  it  has 
been  inspected  and  passed  by  whoever  is  in  charge  of 
the  shop.  Should  a  strand  become  broken  then  the 
cable  must  be  replaced  by  another  one.    Control  cables 


19 

have  a  way  of  wearing  out  and  fraying  wherever  they 
pass  round  pulleys.  Every  time  an  airplane  comes 
down  from  flight  the  mechanic  should  carefully  examine 
the  cables  wherever  they  pass  round  the  pulleys,  and, 
if  he  finds  a  strand  broken,  he  should  report  that  fact 
at  once. 

The  ailerons  balance  cable  on  the  top  of  the  top  plane 
is  often  forgotten,  since  it  is  necessary  to  fetch  a  step 
ladder  to  examine  it.    DON'T  OVERLOOK  THIS 


Wif^f  Loops 


Wron^  Kif?d  oi  Wire  Loop  f'XQ^     \  O 

3^  Pefor/z>a-fiotp  o-f  Wron^  .  '  '~ 

Corree/-  F^orr/r  of  Wire  Loo/^ 

ADJUSTMENTS. 

^  Control  Surfaces.— The  greatest  care  must  be  exer- 
cised in  properly  rigging  the  aileron,  rudder  and  ele- 


20 


vator,  for  the  pilot  entirely  depends  upon  them  in 
managing  the  airplane  (see  Fig.  14). 

The  ailerons  and  elevators  should  be  rigged  so  that 
WHEN  THE  MACHINE  IS  IN  FLIGHT  they  are 
in  a  fair  true  hne  with  the  surface  in  front  and  to  which 
they  are  hinged.  If  the  surface  to  which  they  are 
hinged  is  NOT  A  LIFTING  SURFACE,  then 
rig  the  controlling  sui'face  to  be  in  a  fair  true  line  with 
the  surface  in  front.  If  the  controlling  surface  is 
hinged  a  little  below  what  it  would  be  if  it  was  in  a  fair 
true  line  with  the  surface  in  front.  This  is  because  in 
such  a  case  it  is  set  at  an  angle  of  incidence.  This 
angle  will,  when  the  machine  is  flying,  produce  lift 
and  cause  it  to  lift  a  little  above  the  point  at  which  it 
has  been  rigged  on  the  ground.  It  is  able  to  hft  owing 
to  a  certain  amount  of  slack  in  the  control  wu-e  holding 
it — and  you  can't  adjust  the  control  wire  to  have  no 


slack,  because  that  would  cause  it  to  bind  against  the 
pulleys  and  make  the  operation  of  it  too  hard  for  the 
pilot.  It  is,  therefore,  necessary  to  rig  it  a  little  below 
what  it  would  be  if  it  was  rigged  in  a  fair  true  line  with 
the  surface  in  front.  Remember  that  this  only  applies 
when  it  is  hinged  to  a  lifting  surface.  The  greater  the 
angle  of  incidence  of  the  lifting  surface  in  front,  then 
the  more  the  controlling  surface  will  have  to  be  rigged 
down.  As  a  general  rule  you  will  be  safe  in  rigging  it 
down  so  that  the  trailing  edge  of  the  controlling  surface 
is  ^  to  I  of  an  inch  below  where  it  would  be  if  it  was  in 
a  fair  true  hne  with  the  surface  front — or  half  an  inch 
down  for  every  18  inches  of  chord  of  the  controlling 
surface. 


21 

When  adjusting  the  controlling  surfaces  the  pilot's 
control  levers  must  be  in  a  neutral  position.  It  is  not 
sufficient  to  lash  them  in  that  position.  They  should 
be  blocked  into  position. 

Remember  that  controlling  surfaces  must  never  be 
adjusted  with  a  view  to  altering  the  stability  of  the 
machine.  Nothing  can  be  accomplished  in  that  way. 
The  only  result  will  be  that  the  control  of  the  airplane 
will  be  spoiled. 

CONTROL  CABLES. 

The  adjustment  of  the  control  cables  is  quite  an  art, 
and  upon  it  will  depend  to  a  large  degree  the  quick  and 
easy  control  of  the  airplane  by  the  pilot.  The  method 
is  as  follows: 

After  having  rigged  the  controlling  surfaces  reniove 
the  blocking  which  has  kept  the  control  levers  rigid. 
Then  sitting  in  the  pilot's  seat,  move  the  control  levers 
smartly.  Tension  up  the  control  cables  so  that  when 
the  levers  are  smartly  moved  there  is  no  perceptible 
snatch  or  lag.  Be  careful  not  to  tension  up  the  cables 
more  than  necessary  to  take  out  the  snatch.  If 
you  tension  them  too  much  the  cables  will  bind  round 
the  pulleys  and  result  in  hard  work  for  the  pilot  and 
also  in  throwing  dangerous  stresses  upon  the  controlUng 
surfaces,  which  are  sometimes  of  rather  flimsy  construc- 
tion. It  will  also  cause  the  cables  to  fray  round  the 
pulleys  quicker  than  would  otherwise  be  the  case. 

Now,  after  having  tensioned  the  cable  sufficiently 
to  take  out  the  snatch  or  lag,  place  the  levers  in  their 
neutral  position  and  move  them  backwards  and  for- 
wards not  more  than  eighth  of  an  inch  either  side  of  the 
neutral  position.  If  the  adjustment  is  correct  you 
should  be  able  to  see  the  controlling  surfaces  move. 
If  they  do  not  move  then  the  control  cables  are  too 
slack. 

FLYING  POSITION. 

Before  rigging  the  machine  it  is  necessary  to  place  . 
it  in  what  is  known  as  its  "jflying  position." 

In  the  case  of  an  airplane  fitted  with  a  stationary 
engine  this  is  best  secured  by  blocking  up  the  machine 


22 

so  that  the  engine  foundations  are  perfectly  horizontal 
both  LONGITUDINALLY  and  LATERALLY.  This 
is  done  by  placing  a  straight  edge  and  a  spirit  level  on 
the  engine  foundations,  and  you  must  be  very  careful 
indeed  that  the  bubble  is  exactly  in  the  center  of  the 
level.  The  sUghtest  error  will  be  much  magnified 
towards  the  wing  tips  and  tail.  Great  care  should  be 
taken  to  block  the  machine  up  RIGIDLY.  In  case  it 
gets  accidentally  distm'bed  during  the  rigging  of  the 
machine,  you  should  constantly  verify  the  flying  posi- 
tion by  running  the  straight  edge  and  the  spirit  level 
over  the  engine  foundations.  CAREFULLY  TEST 
THE  STRAIGHT  EDGE  FOR  TRUTH  BEFORE 
USING  IT,  for,  being  usually  made  of  wood,  it  will 
not  remain  true  long.  Place  it  lightly  in  a  vise,  and  in 
such  a  position  that  a  spirit  level  on  top  shows  the 
bubble  exactly  in  the  center.  Now  slowly  move  the 
level  along  the  straight  edge.  The  bubble  should 
remain  exactly  in  the  center.  If  it  does  not,  then  the 
straight  edge  is  not  true,  and  must  be  corrected. 
NEVER  OMIT  DOING  THIS. 

Angle  of  Incidence. 

One  method  of  finding  the  angle  of  incidence  is  as 
follows  (see  Fig.  15). 

The  corner  of  the  straight  edge  must  be  placed  under- 
neath and  against  the  CENTER  OF  THE  REAR 
SPAR  and  held  in  a  horizontal  position  parallel  to  the 
ribs.  This  is  secured  by  using  a  spirit  level.  The  set 
measurements  for  the  angle  of  incidence  will  then  be 
from  the  top  of  the  straight  edge  to  the  CENTER  of 
the  bottom  surface  of  the  main  spar,  or  it  may  be  from 
the  top  of  the  straight  edge  to  the  lowest  part  of  the 
leading  edge.  Be  careful  to  take  the  adjustment  from 
the  center  of  the  spar  and  to  see  that  the  bubble  is 
exactly  in  the  center  of  the  level.  Remember  that  all 
this  will  be  useless  if  the  machine  has  not  been  accu- 
rately placed  in  its  "flying  position." 

This  method  of  finding  the  angle  of  incidence  must 
be  used  under  every  part  of  the  plane  where  struts 
occur.  The  method  should  not  be  used  midway 
between  the  struts  because,  in  such  a  place,  the  spars 


23 

may  have  taken  a  slight  permanent  set  up  or  down — 
not  sufficiently  bad  to  make  any  material  difference 
to  the  fljing  of  the  machine,  but  quite  bad  enough  to 
throw  out  the  angle  of  incidence  adjustment. 

If  the  angle  of  incidence  is  not  right,  correct  it  as 
follows: 

If  it  is  too  gi'eat  then  the  rear  spar  must  be  warped 
up  until  it  is  right,  and  this  is  done  bv  slacking  off  ALL 
THE  WIRES  going  to  the  top  of  the  struts  and 
then  tightening  ALL  THE  WIRES  going  to  the  bot- 


F-zg.  15. 


tom  of  |the  [struts.  If  the  angle  is  less  [than'Jit 
should  be  then  slack  off  ALL  THE  WIRES  going  to 
the  bottom  of  the  struts  and  tighten  ALL  THE 
WIRES  going  to  the  top  of  the  struts  until  you  have 
the  correct  adjustment.  Do  not  attempt  to  secure 
the  adjustment  by  merely  adjusting  the  incidence 
■s^dres.  This  is  a  veiy  bad  practice  indeed  and  while, 
owing  to  the  airplane  being  of  such  ffimsy  construction, 
it  may  be  possible  to  change  the  angle  of  incidence  by 


24 


adjusting  merely  the  incidence  wires,  the  result  of  such 
practice  is  to  throw  other  wires  into  undue  tension, 
and  that  will  cause  the  framework  to  become  distorted. 

Dihedral  Angle. 

One  method  of  securing  the  dihedral  angle,  which  is 
the  upward  inclination  of  the  wings  towards  their  tips, 
is  as  follows;  this  method  will,  at  the  same  time,  give 
you  the  angle  of  incidence  (see  Fig.  16) : 


,  A-  One  Mef&ad  »/  Hotif/^a  Sfr/nj 
B±_finefher  •   


The  strings,  drawn  very  tight,  must  be  taken  over 
both  main  and  rear  spars  of  the  top  plane.  They  must 
run  between  points  on  the  spars  just  inside  the  outer 
struts.  The  set  measurement  is  then  from  the  strings 
down  to  four  points  on  the  main  and  rear  spars  of  the 
the  center  section  plane. 

These  points  should  be  just  inside  the  four  center 
section  struts;  that  is  to  say,  as  far  as  possible  away 
from  the  center  of  the  center  section.  DO  NOT 
ATTEMPT  to  take  the  set  measurement  near  the 
center  of  the  center  section. 

The  string  should  be  as  tight  as  possible,  and,  if  it 
can  be  done,  the  best  way  to  arrange  it  is  as  shown  in 
the  diagram,  i.  e.,  by  weighting  the  string  down  to  the 
spars  by  means  of  weights.  This  will  give  a  tight  and 
MOTIONLESS  string.  Do  not  use  the  method  shown 
in  the  left  side  of  the  diagram,  if  the  weight  on  the  end 


25 

of  the  string  causes  the  wing  tip  to  distort.  However 
careful  the  above  adjustment  is  made  there  is  almost 
sure  to  be  some  slight  error,  and  it  is  necessary  to  take 
certain  check  measurements,  as  follows: 

Each  bay  must  be  diagonally  measured,  and  such 
diagonal  measurements  must  be  the  same  on  each  side 
of  the  airplane.  As  a  rule  these  diagonal  measure- 
ments are  taken  from  the  bottom  sockets  of  one  strut 
to  the  top  socket  of  another  strut,  but  this  is  all  wrong 
because  of  possible  inaccuracies  due  to  faulty  manu- 
facture. The  points  between  which  the  diagonal 
measurements  are  taken  must  be  at  fixed  distances  from 
the  butt  of  the  spars,  such  fixed  distances  being  exactly 
the  same  on  each  side  of  the  machine,  thus  (see  Fig.  17) : 


"     J 

P 

^^^^ 

3/ 

7 

Piaaonal    1  Mos^  f-qual  Q- 

^   -  - 

Pis-lanct  from  fl  fa  B<0Mosf  j-t^ual  Pisfartce  V  io  Boff. 

— 1  •    £  c  -  ■• 

»e+h    F-rcn/  4  K^tar  Bays  hfloff  ChecktJ  . 


Diagonal  1  must  =  4. 
Diagonal  2  must  =  3. 

Distance  from  A  to  Butt  must  =  distance  from  D  to 
Butt. 

The  same  applies  to  B  and  C  and  to  E  and  F. 

Both  Front  and  Rear  Bays  must  be  checked. 

It  would  be  better  still  to  use  the  center  line  of  the 
fuselage  instead  of  the  butts  of  the  spars,  but  for  the 
fact  that  such  method  is  a  troublesome  one. 

Dihedral  Board. 

Another  method  of  securing  the  dihedral  angle  (and 
also  the  angle  of  incidence)  is  by  means  of  the  dihedral 


26 

board.  The  dihedral  board  is  a  Ught  handy  thing  to 
use  but  leads  to  many  errors,  and  should  not  be  used 
unless  necessary.    The  reasons  are  as  follows: 

The  dihedral  board  is  probably  not  true.  If  you 
MUST  use  it,  then  be  very  careful  to  test  it  for  truth 
beforehand.  Another  reason  against  its  use  is  that 
you  have  to  use  it  on  the  spars  BETWEEN  THE 
STRUTS,  and  that  is  just  where  the  spars  may  have  a 
Uttle  permanent  set  up  or  down  which  will,  of  course, 
throw  out  the  accm-acy  of  the  adjustment.  Then  again, 
there  may  be  inequaUties  of  surface  on  the  spar  due  to 
faulty  manufacture.  The  method  of  using  it  is  as 
follows  (see  Fig.  18) . 


F-16.  16. 

If  the  dihedral  board  is  used  then  the  bays  must  be 
carefully  diagonally  measured  as  explained  above. 
Whatever  the  method  is  used  to  secure  the  angle  of 
incidence,  be  sure  that,  after  the  job  is  done,  THE 
SPARS  ARE  PERFECTLY  STRAIGHT. 

STAGGER. 

The  stagger  is  the  distance  the  top  plane  is  in  advance 
of  the  bottom  plane  when  the  machine  is  in  flying  posi- 
tion. The  set  measurement  is  obtained  as  follows  (see 
Fig.  19). 

A,  set  measurement  when  taken  along  Prolongation 
of  Chord. 

B,  set  measurement  when  taken  along  Horizontal 
Line. 

Plumb  lines  must  be  dropped  over  the  leading  edges 
wherever  struts  occur,  and  also  near  the  fuselage.  The 
set  measurement  is  taken  from  the  front  of  the  lower 


27 

leading  edge  to  the  plumb  line.  Remember  that  it 
makes  a  difference  whether  you  measure  along  a  hori- 
zontal line  (which  can  be  found  by  using  a  straight- 
edge and  a  spirit  level)  or  along  a  projection  of  the 
chord.  The  correct  line  along  which  to  measure  is 
laid  down  in  the  rigging  diagram  accompanying  each 
machine. 

If  you  make  a  mistake  and  measm-e  along  the  wrong 
line  this  may  make  a  difference  of  a  quarter  of  an  inch 
or  more  to  the  stagger,  with  the  certain  result  that  the 
airplane  will  be  nose  heavy  or  tail  heavy. 

A'  Se^  Measuremer-/   W/^e^  Taker?  fffof7^    ProfoA>^a-f/ov  ef  Chofe^ 


When  the  adjustment  of  the  angles  of  incidence, 
dihedral  angle,  and  the  stagger  have  been  secured 
RUN  OVER  ALL  OF  THEM  AGAIN  AS,  IN 
MAKING  YOUR  LAST  ADJUSTMENT,  YOU  MAY 
POSSIBLY  HAVE  THROWN  OUT  YOUR  FIRST 
ONE. 

OVER  ALL  ADJUSTMENTS. 

Now  you  should  take  the  following  over-all  measure- 
ments (see  Fig.  20). 


28 

The  straight  lines  "A"  and  "C"  must  be  equal. 
The  point  "B"  is  the  center  of  the  propeller  thrust,  or, 
in  the  case  of  a  "pusher"  machine,  the  center  of  the 
nacelle.  The  points  "D"  and  "E"  are  marked  on 
the  main  spar;  and  must  in  each  be  the  same  distance 
from  the  butt  of  the  spar.    Do  not  attempt  to  make 


3 


"D"  and  "E"  merely  sockets  on  the  outer  struts  as 
they  may  not  have  been  placed  quite  accurately  by 
the  manufacturers.  The  lines  "A"  and  "C"  must  be 
taken  from  both  top  and  bottom  spars— two  measure- 
ments on  each  side  of  the  airplane.  ^ 

Now  measure  the  distance  between    I*    and  U 
and  "H"  and  "G."    These  two  measurements  must 


29 

be  equal.  "G"  is  the  center  of  the  fuselage  or  rudder 
post.  "F"  and  "H"  are  points  marked  on  both  top 
and  bottom  rear  spars,  and  in  each  case  must  be  the 
same  fixed  distance  from  the  butt  of  the  spar.  Remem- 
ber that  "F"  and  "H"  must  not  be  taken  to  be  the 
sockets  of  the  outer  rear  struts.  Two  measurements 
on  each  side  of  the  akplane.  If  these  over-all  measure- 
ments are  not  correct  then  it  is  probably  due  to  some 
of  your  drift  or  anti-drift  wires  being  too  tight  or  too 
slack.  It  may  possibly  be  due  to  the  fuselage  being 
out  of  truth,  but,  of  course,  you  should  have  made 
quite  sure  that  the  fuselage  was  true  before  rigging  the 
rest  of  the  machine.  Again,  it  may  be  due  to  the 
internal  bracing  wires  not  being  accurately  adjusted 
but,  of  course,  that  should  have  been  done  before 
covering  the  planes  with  fabric. 

PROPELLERS. 

The  last  thing  to  go  on  to  the  machine  is  usually 
the  propeller,  by  which  time  there  is  usually  a  rush  to 
get  the  machine  out.  You  must,  however,  be  very 
careful  to  see  that  this  is  fitted  on  true  and  straight. 
This  is  easily  verified  by  bringing  the  tip  of  one  blade 
round  to  graze  some  fixed  object  such  as  a  trestle. 
Mark  the  place  where  the  tip  of  the  blade  touches  it. 
Now  bring  the  tip  of  the  other  blade  round  and  it 
should  be  within  an  eighth  of  an  inch  of  the  mark. 
If  it  is  not  so,  then  it  is  probably  due  to  some  of  the 
propeller  bolts  being  pulled  up  too  tight.  It  may  be 
due  to  the  propeller  itself  not  being  true. 

FUSELAGE. 

The  methods  of  trueing  fuselages  are  laid  down  in 
the  rigging  diagrams  accompanying  each  machine. 
After  having  adjusted  the  fuselage  according  to  the 
specified  directions,  then  arrange  it  on  trestles  in  such 
a  way  as  to  make  about  three-quarters  of  the  fuselage 
towards  the  tail  stick  out  unsupported.  In  this  way 
you  wiU  get  as  near  as  possible  to  flying  conditions 
and,  when  it  is  in  this  position,  you  should  run  over  the 
adjustments  again.    If  this  is  not  done  the  fuselage 


30 

may  be  out  of  truth  but  perhaps  appear  all  right  when 
supported  by  trestles  at  both  ends  as,  in  such  case, 
its  weight  may  keep  it  true  as  long  as  it  is  resting  upon 
the  trestles. 

TAIL  PLANE. 

The  exact  angle  of  incidence  of  the  tail  plane  is  given 
in  the  rigging  diagram.  Be  careful  to  see,  however, 
that  the  spars  are  horizontal.  If  they  are  tapered 
spars  then  see  that  their  center  lines  are  horizontal. 
After  the  tail  plane  has  been  rigged,  support  the  machine 
so  that  the  tail  is  unsupported  as  explained  above. 
Then  verify  the  adjustment  and  make  sure  that  the 
tail  plane  spars  are  horizontal  when  the  machine  is  in 
flying  position. 

RUDDER,  AILERONS,  ELEVATOR. 

These  controlling  surfaces  must  not  be  distorted  in 
any  way.  If  they  are  held  true  by  bracing  wires  then 
such  wires  must  be  carefully  adjusted.  If  they  are 
distorted  and  there  are  no  bracing  wires  with  which 
to  true  them  up,  then  the  matter  should  be  reported,  as 
it  may  be  necessary  to  replace  some  of  the  internal 
framework. 

LANDING  GEAR. 

The  landing  gear  must  be  very  carefully  aligned  as 
laid  down  in  the  rigging  diagram  accompanying  each 
machine. 

1.  Be  very  careful  to  see  that  the  landing  gear  struts 
bed  well  down  into  their  sockets.  If  this  is  not  done 
then  after  having  a  few  rough  landings,  they  will  bed 
down  farther  and  thi'ow  the  landing  gear  out  of  align- 
ment, with  the  result  that  the  machine  will  not  taxi 
straight. 

2.  When  rigging  the  landing  gear  the  airplane  must 
be  blocked  up  in  its  flying  position  and  sufficiently 
high  so  that  wheels  are  off  the  ground.  When  in  this 
position  the  axle  must  be  horizontal. 

3.  Be  very  careful  to  see  that  the  shock  absorbers 
are  of  equal  tension,  and  that  the  same  length  of  elastic 
and  the  same  number  of  turns  are  used  in  the  case  of 
each  absorber. 


31 

HANDLING  OF  AIRPLANES. 

An  extraordinary  amount  of  damage  is  done  by  the 
mishandling  of  airplanes  and  in  blocking  them  up  from 
the  ground  in  the  wrong  way.  The  golden  rule  to 
observe  is: 

Produce  No  Bending  Stresses. 

1.  Remember  that  nearly  all  the  wood  of  the  airplane 
is  designed  to  take  the  stress  of  direct  COMPRESSION 
and  it  cannot  be  safely  bent.  In  blocking  up  an  air- 
plane from  the  ground  the  blocking  must  be  used  in 
such  a  way  as  to  come  underneath  the  interplane 
struts  and  the  fuselage  struts.  Padding  should  always 
be  placed  on  the  points  upon  which  the  airplane  rests. 

2.  When  puUing  the  machine  along  the  ground 
always,  if  possible,  puU  from  the  landing  gear.  If 
necessary  to  pull  from  elsewhere  then  do  so  by  grasping 
the  interplane  struts  as  low  down  as  possible. 

3.  As  regards  handling  parts  of  airplanes.  Never 
lay  anything  covered  with  fabric  on  a  concrete  floor, 
as  any  slight  movement  will  cause  the  fabric  to  scrape 
over  the  concrete  with  resultant  damage. 

4.  Struts,  spars,  etc.,  should  never  be  left  about  the 
floor,  as  in  such  position  they  are  likely  to  become  dam- 
aged, and  it  has  akeady  been  explained  how  necessary 
it  is  to  protect  the  outside  fiber  of  the  wood.  Remem- 
ber also  that  wood  easily  becomes  distorted.  This 
particularly  appUes  to  the  interplane  struts.  The  best 
method  is  to  stand  them  up  in  as  nearly  vertical  posi- 
tion as  possible. 

NOTES  FOR  KEEPING  THE  AIRPLANE  IN 
GOOD  CONDITION. 

Cleanliness. — The  fabric  must  be  kept  clean  and  free 
from  oil,  as  that  will  rot  it.  To  rake  out  dirt  or  oily 
patches  try  acetone.  If  that  will  not  do  the  job  try 
gasohne,  but  use  it  sparingly  or  otherwise  it  will  take 
off  an  unnecessary  amount  of  dope.  If  that  wiU.  not 
remove  it,  then  hot  water  and  castile  soap  will  do  so. 
Use  water  sparingly,  as  otherwise  it  may  get  inside  the 
planes  and  rust  the  internal  bracing  wires,  or  cause  some 
of  the  wooden  framework  to  swell. 


32 

The  wheels  of  the  landing  gear  have  a  way  of  throwing 
up  a  great  deal  of  mud  onto  the  lower  plane.  This 
should  be  taken  off  at  once.  Do  not  allow  it  to  dry 
and  do  not  try  to  scrape  it  off  when  dry.  If  dry  then  it 
must  be  moistened  first  as  otherwise  the  fabric  will  be 
spoiled. 

Controlling  Wires. — After  every  flight  pass  your  hand 
over  the  control  wires  and  carefully  examine  them  near 
pulleys.  If  only  one  strand  is  broken  the  wire  must  be 
changed.  Don't  forget  the  aileron  balance  wire  on  the 
top  plane. 

Once  a  day  try  the  tension  of  the  control  wires  by 
smartly  moving  the  control  levers  about  as  explained 
elsewhere.  . 

Wires. — See  that  all  wires  are  kept  well  greased  or 
oiled,  and  that  they  are  all  in  the  same  tension.  When 
examining  your  wires  be  sure  to  have  the  machine  on 
level  ground  as  otherwise  it  may  get  twisted,  throwing 
some  wires  into  undue  tension  and  slackening  others. 
The  best  way,  if  you  have  time,  is  to  block  the  machine 
up  into  its  flying  position. 

If  you  see  a  slack  wire  do  not  jump  to  the  conclusion 
that  it  must  be  tensioned,  perhaps  its  opposition  wire 
is  too  tight,  in  which  case  slacken  it  and  possibly  you 
will  find  that  will  tighten  the  slack  wire. 

Carefully  examine  all  wu'es  and  their  connections 
near  the  propeller,  and  be  sure  that  they  are  snaked 
round  with  safety  wire,  so  that  the  latter  may  keep 
them  out  of  the  way  of  the  propeller  if  they  come  adrift. 

Distortion. — Carefully  examine  all  sm-faces,  including 
the  controlling  surfaces,  to  see  whether  any  distortion 
has  occurred.  If  the  distortion  can  be  corrected  by 
the  adjustment  of  wires  well  and  good,  but  if  not  then 
report  the  matter. 

Adjustment. — Verify  the  angle  of  incidence,  the 
dihedral  angle,  the  stagger,  and  the  over-all  measure- 
ments as  often  as  possible. 

Landing  Gear. — Constantly  examine  the  alignment 
and  fittings  of  the  landing  gear,  and  the  condition  of 
axle,  tires,  shock  absorbers,  wheels  and  the  tail  skid. 

Control  Surfaces. — As  often  as  possible  verify  the 
rigging  position  of  the  ailerons  and  elevator. 


33 

Locking  Arrangements. — Constantly  inspect  the  lock- 
ing arrangements  of  all  tiu-nbuckles,  bolts,  etc. 

Outside  Position. — The  airplane,  when  outside  its 
shed,  must  always  stand  FACING  THE  WIND.  If 
this  is  not  so,  then  the  wind  may  catch  the  controlling 
surfaces  and  move  them  sharply  enough  to  darnage 
them.  If  the  airplane  must  be  moved  dm^ing  windy 
weather  then  the  control  levers  should  be  lashed  fast. 

Sighting. — ^^Tienever  you  have  the  opportunity, 
practice  sighting  one  strut  against  another  to  see  that 
they  are  parallel.  Standing  in  front  of  the  machine, 
which,  in  such  a  case,  should  be  on  level  ground,  sight 
the  center  section  plane  against  the  tail  plane  and  see 
that  the  latter  is  in  line.  Sight  the  leading  edge  against 
the  main  spars,  the  rear  spars  and  the  traihng  edges, 
taking  into  consideration  the  "wash-in"  or  the  "wash- 
out." You  will  be  able  to  see  the  shadow  of  the  spars 
through  the  fabric.  By  practising  this  sort  of  thing 
you  wiU,  after  a  time,  become  quite  expert,  and  will 
be  able  to  diagnose  by  eye  faults  in  efficiency,  stabiUty, 
and  control. 

PROPELLERS. 

The  sole  object  of  a  propeller  is  to  produce  THRUST. 
The  thrust  overcomes  the  drift  of  the  airplane,  and 
draws  ;or  pushes  the  airplane  through  the  air.  The 
thrust  must,^be  equal  to  the  drift  of  the  airplane  at 
flying  speed.  If  it  is  not  equal  to  the  drift  then  the 
airplane  cannot  secure  its  proper  speed.  The  thrust 
will  be  badly  affected  if  any  of  the  following  conditions 
are  not  as  they  should  be  (see  Fig.  21) : 

Pitch-Angle. — The  propeller  screws  through  the  air 
and  its  blades  are  therefore  set  at  an  angle.  This  angle 
is  known  as  the  "pitch-angle"  and  it  must  be  correct 
to  HALF  A  DEGREE.  It  is,  of  course,  smaUer 
towards  the  tips  of  the  blades  just  as  in  the  case  of  the 
pitch-angle  of  a  screw. 

Pitch. — The^pitch  is  the  distance  the  propeller  will 
screw  the  air  in  one  revolution,  supposing  the  air  to 
be^soHd.  As  a  matter  of  fact  the  air  is  not  soHd,  and 
gives  back  to  the  thrust  of  the  propeller  blade  so  that 
the  propeller  does  not  travel  its  full  pitch.  Such 


34 

"give-back"  is  known  as  "slip."  For  instance,  the 
pitch  of  the  propeller  may  be  perhaps  10  feet,  and  the 
propeller  may  have  a  slip  of  2  feet.  The  propeller 
would  then  be  said  to  have  20%  slip. 


To  test  the  pitch-angle  the  propeller  is  mounted  on 
a  shaft,  the  latter  being  mounted  upon  and  at  right 
angles  to  a  beam.  The  fac  of  the  beam  must  be  per- 
fectly straight  and  true. 

Now  select  a  spot  some  distance  (say  about  2  feet) 
from  the  center  of  the  propeller  and,  by  means  of  a 
protractor,  find  the  angle  the  chord  of  the  blade  makes 
with  the  beam.  Then  lay  out  the  angle  out  on  paper 
thus  (see  Fig.  22) : 


X  '  Jln^/e  of  hc>def7ce    as  /rr  f^,g.  dO.  ^  ^  ^  ' 

•  Pi'am.  (Se/ec^ed  per  Ca  /tu/a//o/y)  X 3  It/f, 
C  '  Len^fb  of  Line  C  is  fhe  F,^ch  of  Pro^e//fr 

X  =  Angle  of  incidence  as  in  first  diagram. 
XX  =  Diameter  (selected  for  calculation)  X  3.1417. 
C :  Length  of  line  C  is  the  pitch  of  propeller. 
The  line  marked  CHORD  represents  the  chord  of 
the  propeller.    The  line  marked  CIRCUMFERENCE 


35 

represents  the  face  of  the  beam.  The  angle  the  two 
lines  make  is  the  angle  you  have  found  by  means  of 
the  protractor. 

We  will  suppose,  for  the  sake  of  example,  that  the 
point  at  which  you  have  taken  the  angle  if  2  feet,  from 
the  center  of  the  propeller.  Find  the  cu'cumference 
at  the  point  by  doubling  the  2  feet  (which  is  the  radius) 
and  then  multipljdng  the  result  by  3.1417,  thus: 

(2  feet  X2  feet)  X3. 1417 -12.5668  feet,  i.  e.,  the  cir- 
cumference at  that  part  of  the  propeller.  Bring  it 
down  in  scale,  and  mark  it  off  from  the  point  "A"  and 
along  the  circumference  line.  -Now  draw  the  line 
marked  pitch  from  "B"  (the  end  of  the  circumference 
measurement  of  12.5668  feet)  and  at  right  angles  to  the 
circumference  line. 

Weigh/      Boli  Hol<is  Shctf^  of?  3a//  3earfhas. 


THE  DISTANCE  FROM  "B"  TO  THE  CHORD 
LINE  IS  THE  PITCH  OF  THE  PROPELLER  at 
that  point. 

It  must  agree  with  the  specified  pitch  of  the  propeller 
which  should  be  marked  on  the  hub.  If  it  does  not  do 
so  then  the  pitch-angle  is  wrong.  This  may  be  due 
(1)  to  the  propeller  blade  being  distorted;  (2)  to  faulty 
manufacture;  or  (3)  to  the  hole  through  the  boss  of  the 
propeller  being  out  of  place. 

Degree  of  Error  Allowed. — You  may  allow  an  error 
up  to  half  a  degree  more  or  less  of  the  correct  angle,  but 
if  it  is  greater  than  that  you  must  report  the  matter. 

The  propeller  should  be  tested  as  explained  above 
at  points  along  the  blades,  the  first  point  about  2  feet 
from  the  center  of  the  boss  and  the  others  about  a 
footapart. 


36 


Length. — The  propeller  should  be  carefully  tested  to 
make  sure  the  blades  are  of  equal  length.  There  should 
not  be  a  difference  of  more  than     of  an  inch. 

Balance. — The  prevaiUng  method  of  testing  for 
balance  is  as  follows:  Mount  it  upon  a  shaft.  The 
shaft  must  be  on  ball  bearings.  Place  the  propeller 
in  a  horizontal  position,  and  it  should  remain  in  that 
position.  If  a  weight  of  a  trifle  over  an  ounce  placed 
in  a  bolt-hole  on  one  side  of  the  boss  fails  to  disturb 
the  balance  then  the  propeller  is  unfit  for  use  (see 


The  above  method  does  not,  however,  test  for  the 
balance  of  centrifugal  force,  which  comes  into  play  as 
soon  as  the  propeller  revolves. 


The  test  for  centrifugal  balance  is  as  follows  (see 
Fig.  24): 


The  propeller  must  be  kept  horizontal,  and  while 
in  that  position,  weighed  at  any  fixed  points,  such  as 
A,  B,  C,  D,  E,  and  F,  and  the  weights  noted.  Now 
reverse  the  propeller  and  weigh  at  each  point  again. 
Note  the  result.  The  first  series  of  weights  should 
correspond  to  the  second  series,  thus: 

Weight  A  should  equal  weight  F. 
Weight  B  should  equal  weight  E. 
Weight  C  should  equal  weight  D. 


Fig.  23). 


A    3  C 


37 


There  is  no  official  ruling  as  to  the  degree  of  error 
allowed,  but  if  there  is  any  appreciable  difference  the 
propeller  is  unfit  for  use.  The  points  A,  B  and  C  must, 
of  course,  be  exactly  the  same  distance  from  the  center 
of  the  propeller  as  the  points  D,  E  and  F. 

Surface  Area. — The  surface  area  of  the  blades  should 
be  equal.    Test  with  calipers,  thus  (see  Fig.  25): 


P/s^a/TCe  fi'3  Shoo/d  £^<^ua/ 

K-L 

c-p 

0-H 

Distance  A-B  should  =  K-L. 
Distance  C-D  should  =  I-J. 
Distance  E-F  should  =  G-H. 

There  is  no  official  ruling  as  to  the  degree  of  error 
allowed.  If,  however,  there  is  an  en-or  of  over  \  inch, 
the  propeller  is  really  unfit  for  use.  The  points  A,  B 
C,  D,  E,  and  F  must,  of  course,  be  exactly  the  same 
distance  from  the  center  of  the  propeller  as  the  points 
G,  H,  I,  J,  K,  and  L. 

Camber  {Curvature). — The  camber  of  the  blades 
should  (1)  be  equal;  (2)  it  should  decrease  evenly 
towards  the  tips  of  the  blades,  and  (3)  its  greatest 
depth  should,  at  any  point  of  the  blade,  be  at  about  the 
same  proportion  of  the  chord  from  the  leading  edge,  as 
at  other  points. 

It  is  difficult  to  test  the  top  camber  without  a  set  of 
templates,  but  a  fairly  accurate  idea  of  the  concave 
camber  underneath  the  blade  can  be  sucured  by  slowly 
passing  a  straight-edge  along  the  blade — the  straight- 
edge (a  steel  rule  will  do)  being  held  at  right  angles 
to  the  length  of  the  blade  and  touching  both  leading  and 
traihng  edges,  thus  (see  Fig.  26). 


38 


The  concave  cui-vature  can  now  easily  be  seen,  and, 
as  you  pass  the  straight-edge  along  the  blade,  you 
should  look  out  for  any  irregularities  of  the  curvature 
which  should  gradually  and  evenly  decrease  towards 
the  tip  of  the  blade. 

Straightness. — To  test  for  straightness  mount  the 
propeller  on  a  shaft.  Now  bring  the  tip  of  one  blade 
to  graze  some  fixed  object.  Mark  the  point  it  grazes. 
Now  bring  the  other  tip  round  and  it  should  come 
within  I  inch  of  the  mark.  If  it  does  not  do  so  it  is 
due  (1)  to  the  propeller  being  distorted,  or  (2)  to  the 
hole  through  the  boss  being  out  of  place.  In  either 
case  it  is  unfit  for  use. 

The  Joints. — -The  method  of  testing  the  glued  joints 
is  by  revolving  the  propeller  at  5  to  10  per  cent  greater 
speed  than  it  will  be  called  upon  to  make  in  flight,  and 
then  carefully  examining  the  joints  to  see  if  they  have 
opened.  It  is  not  likelj%  however,  that  you  will  have 
the  opportunity  of  making  this  test.  You  should, 
however,  examine  all  the  glued  joints  very  carefully, 


F-iG.  2.6. 

trying  by  hand  to  see  if  they  are  quite  sound.  Suspect 
the  propeller  in  which  the  joints  appear  to  hold  any 
thickness  of  glue.  Sometimes  the  joints  in  the  boss 
open  a  little,  but  this  is  not  dangerous  unless  they 
extend  to  the  blades  as  the  bolts  will  hold  them 
together. 

Condition  of  Surface. — The  surface  should  be  per- 
fectly smooth,  especially  towards  the  tips  of  the  blades. 
Some  propeller  tips  have  a  speed  of  over  30,000  feet  a 
minute,  and  any  roughness  will  produce  a  bad  drift  or 
resistance  and  reduce  the  efficionc}^  of  the  propeller. 


39 


Mounting  the  Propeller. — Be  careful  to  see  that  the 
propeller  is  mounted  quite  straight  on  the  shaft. 
Bring  the  tip  of  one  blade  round  to  graze  some  fixed 
object.  Mark  the  point  it  grazes.  Now  bring  the 
tip  of  the  other  blade  round  to  the  mark,  and  it  should 
be  within  |  of  an  inch  of  it.  If  it  is  not  within  |  inch 
of  the  mark  it  is  due  to  either  the  propeller  being  faulty 
in  which  case  it  should  not  be  used,  or  it  may  be  due  to 
some  of  the  propeller  bolts  being  too  slack  or  others 
being  pulled  up  too  tight. 

Care  of  Propellers. — ^The  care  of  propellers  is  of  the 
greatest  importance,  as  they  are  very  likely  to  distort 
and  lose  their  correct  pitch-angle  and  straightness. 

1.  Do  not  store  them  in  very  damp  or  very  dry  place. 
2  Do  not  store  them  where  the  sun  will  shine  upon 
them. 

3.  Never  leave  them  long  in  a  horizontal  position  or 
leaning  up  against  a  wall. 

4.  They  should  be  hung  on  pegs,  the  latter  at  right 
angles  to  the  wall,  and  the  position  of  the  propeller 
should  be  vertical. 

If  these  points  are  not  observed,  you  may  be  sure  of 
the  following  results : 

1.  Lack  of  efficiency,  resulting  in  less  airplane  speed 
and  climb  than  would  otherwise  be  the  case, 

2.  Propeller  "flutter,"  i.  e.,  vibration,  which  will 
cause  the  propeller  to  distort  and  possibly  collapse, 

3.  A  bad  stress  upon  the  engine  shaft  and  its  bearings. 
Note. — If  engine  vibration  is  complained  of  it  may 

be  due  to  faulty  propeller,  or  to  the  propeller  not  being 
straight  on  its  shaft. 


I 


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